Halisarca harmelini sp. nov. is described from the northwestern Mediterranean Sea at depths between 15–65 m from coralligenous rocks. The new species occurs as thin sheets only on the bryozoan Smittina cervicornis. Morphologically, it is characterized by its thin layer architecture, skin-like, lusterless and smooth surface, a soft, very delicate and easily torn texture, and pale-yellow colour. At the cytological level H. harmelini is characterized by a complex of cells with inclusions: spherulous, vacuolar, granular, microgranular and rhabdiferous cells. The new species is clearly different in cell and endobiotic bacteria content and in external morphology from all previously described species of Halisarca. In addition to the description of the new species, we have conducted a thorough ultrastructural investigation and re-described the type species Halisarca dujardini Johnston, 1842 using specimens from different region of the North Atlantic, Arctic and North Pacific. To investigate the relationships between